The diversities in the morphology and morphometric parameters of PM may be attributed to racial disparities or differences in the build of different populations or may be due to differences in the sample size. Comparing the prevalence of the muscle in our study with the findings in other populations, the south Indian population not only showed the lowest frequency of presence but also had an enormous difference from the others (Table 3).
Table 3
The prevalence of the muscle in various populations:
Authors | Year of Study | Sample size | Population | Prevalence (%) |
Ancel [1] | 1902 | 146 | French | 71 |
Loth [19] | 1924 | 200 | Polander | 79 |
Vallois [38] | 1926 | 2266 | African | 82 |
Wagenseil [39] | 1927 | 80 | Chinese | 99 |
Mori [25] | 1964 | 250 | Japanese | 94.5 |
Kaur et al [14] | 2016 | 30 | North Indian | 93.33 |
Das et al [5] | 2017 | 25 | Indian | 72 |
Hojaij et al [12] | 2020 | 30 | Brazilian | 83.33 |
Present study | 2022 | 51 | South Indian | 41.17 |
Sumalatha et al [33] studied the prevalence of PM both in fetuses and adults, the prevalence of PM was 100% in fetuses, whereas in adults it was 87%. The regression of muscle with advancing age may be the reason behind this difference.
Table 3 shows a considerable range of incidences, from 71% in the French population [1] to 99% in the Chinese [39]. Whereas, in this study, the South Indian population sample showed an incidence of 41.17%, which was notably less than the prevalence reported by different authors (Table 2).
Discussing literature makes it prudent that PM is common in human populations of various ethnicities. This augments its credibility as a graft since, in spite of its high prevalence, its absence does not appear to cause any consequential dysfunction. Therefore, purportedly, it can be harvested as a graft with no collateral effects. Additionally, the rectus-pyramidalis sheath and transplants can also be used in hernia repair [27].
In the present study, we did not report any variations in the arterial supply and innervation of the muscle. All were innervated by the nervus subcostalis (T12) and arterial supply was by a branch from the inferior epigastric artery. Knowledge about the neurovascular pedicle of a muscle makes the job easier for surgeons operating on flap procedures [37]. Hence, variations in the arterial supply and innervation are crucial for the PM to be considered as a flap in reconstructive surgeries [13]. Moreover, variations in the innervation patterns of the PM are assumed to be related to the developmental process of the abdominal musculature [11] as well as to the deviation in the lumbar plexus formation [35].
The mean length and width of the PM in this study provide an estimate of the size of the lesions that can be treated with the PM grafts in the south Indian population. Pfannenstiehl incision with insignificant donor site morbidity makes it an appealing donor muscle for microsurgical transfer [29]. Furthermore, the high prevalence of the muscle in the study population may enable its use as a source of stem cells although this possibility is still under further research.
In this sample of the south Indian population, the unilaterality of the PM was exceptionally rare (only one case) (1.96%) than in other populations [5, 8]. Henceforward, we have reported the lowest incidence of unilateral PM as compared to the rest of the world’s population. Despite all statistical data and descriptions collated, the PM’s great variability on morphometry and frequency found in this and other studies, makes it onerous to predict what will be found when incising an individual’s abdomen. The various actions or tasks of the muscles of the anterior abdominal wall and the numerous surgical procedures in the region emphasize the clinical relevance of the muscles in this region. Clinicians should be familiar with possible regional variations to minimize the risk of complications and apply the best method of treatment when they occur. The manifestation of the rectus abdominis muscle and PM crossing the midline can make the procedures more complex [21].
PM, somewhat a variable muscle and thus morphologically very important, was studied by various authors in the Japanese population in detail [1, 22, 25, 36]. In 1964, Mori (1964) [25] noticed various patterns in the mode of insertion or the level of apex and origin of the muscle and thus classified the formation of the muscle into nine types or patterns. In the present study, we have used this classification system to typify the muscle into nine patterns and found type 7 as the most common which was in consonance with the findings of Mori. Although such classification based on morphology is critical in understanding the variational anatomy of the muscle, studies conducted afterwards have mentioned very little or almost nothing about it. In the present study, we have revisited this pioneer classification intending to encourage future anthropologists to pay genuine recognition while studying this musculature.
The morphometry of PM also varies with its frequency. Every single biometric parameter of the PM was greater on the right side than the left side except the angle of insertion which was greater on left (Table 1). The average lengths in both genders were almost half of the mean value reported in the corresponding Greek, Nigerian and Brazillian ethnicity [8, 12, 26] and almost equal to the north Indian population [5, 14]. The difference may be due to racial variations i.e., differences of stature in different populations. On contrary, the average muscle width was pretty near to the values reported in the Nigerian and Greek populations [26, 27] and lesser than the Brazilian and northern Indians [5, 12]. Statistically significant sexual dimorphism was found based on muscle length and width (Table 1).
Since PM serves as a landmark for surgeons in making a midline infraumbilical incision [28, 30], PPI thus calculated could potentially serve as an identifier in limiting the incision [30]. The average PPI was 35.15 ± 4.38% in males and 36.01 ± 4.97% in females in the north Indian population [5] and 38% ± 18 (39% ± 18 males, 35% ± 19 females) in Kenyan populations [15]. The present study done on south Indian populations reported the mean PPI to be 32.82 ± 1.65% in males and 27.50 ± 1.08% in females. It apparently shows that irrespective of ethnicity and body built, the muscle terminates largely within the 2nd quarter (25–50%) of the infra-umbilical linea alba (measured from pubic symphysis) in most of the populations. As PM covers the lower 25–35% of the distance between the umbilicus and pubic symphysis, these values would be useful for surgeons in limiting the upward extension of midline infra-umbilical incisions.
Currently, we have measured the angle between the long axis of the PM and midline joining the umbilicus and symphysis pubis at the level of the insertion of the muscle into the linea alba. The angulation is crucial in determining the functionality and biomechanics of the linea alba. If the angle is acute, then the tangential force of the PM acts more or less in line with the linea alba making it perfectly taught along the midline. However, as the angulation increases, the contraction of the PM pulls the linea alba laterally. In the case of the bilateral presence of PM with an obtuse angle of insertion, bilateral contraction can cause disintegration of both the rectus to the sides, this being accompanied by the extension of the linea alba tissue and bulging of the abdominal wall may result in the infra umbilical herniation of the abdominal viscera [23]. Additionally, subjected to these trends of greater stresses observed in the linea alba in the transverse direction compared to the longitudinal, a few recommendations were made regarding designing the orientation of mesh for infraumbilical hernia repair. Accordingly, the most compliant axis of the biomaterial should be oriented in the craniocaudal (longitudinal) direction, and the strongest axis of the biomaterial should be oriented in the medial-lateral (transverse) direction [6]. To date, no literature has described this biometric parameter, so could not compare our findings with any pre-determined reference value.
The variations in the prevalence of PM and its morphometry are significant during imaging investigation of groin pain to circumvent misunderstanding and interpretation as a mass and unnecessary surgical intervention (Koulouris 2008). The ectopic propagation of an endometrial mass in the PM substance might be spotted in patients with chronic groin pain [7, 9]. Hence, further investigations are required to validate whether the absence or asymmetry of the musculature is a risk factor for obscure groin pain.
Subsequently, we can assert that the PM is still a great conundrum in muscle anthropology. The variability in its prevalence and morphometry (in the present and previous studies) and indifference to its absence perpetuates the uncertainties about its functions [4, 18]. Its phylogenetic origin has not yet been truly elucidated, which makes it difficult to ascertain its developmental process [4]. While some studies say that the PM is a vestigial muscle phylogenetically derived from monotremes thus tending to disappear in the future, others believe it to be a newly formed structure in the primate, reaching its highest development in man [3], others argue that its presence is related to the erect posture [12] and thus tends to become more prevalent.